Bonding materials using polycrystalline magnesium orthosilicate

ABSTRACT

A method of bonding a metal to a ceramic or two ceramic materials including the steps of melting a polycrystalline magnesium orthosilicate compound; and applying the melted polycrystalline magnesium orthosilicate compound between two materials which are metal with a ceramic, or two ceramic materials, or two metallic materials, bonding using the polycrystalline magnesium orthosilicate to bond the two materials together.

CROSS REFERENCE TO RELATED APPLICATION

Reference is made to commonly assigned U.S. patent application Ser. No.09/361,878 filed concurrently herewith, entitled “Making CrystallineMagnesium Orthosilicate” by Chatterjee et al, the disclosure of which isincorporated herein.

FIELD OF THE INVENTION

The present invention relates to a method of bonding two materials usingpolycrystalline magnesium orthosilicate.

BACKGROUND OF THE INVENTION

Magnesium orthosilicate, whose chemical formula is 2MgO—SiO₂ andchemical name is Forsterite is typically used for applications requiringa high coefficient of thermal expansion. The usual procedure formanufacture of this compound is to mix pure forms of magnesia and silicain stoichiometric proportions, then calcining and milling undercontrolled conditions, which can be modified to meet priorityspecifications. Typical applications of magnesium orthosilicates are inmaking substrates, particularly for high frequency electronics, thickfilms, and ceramic-to-metal seals and high temperature bonding orjoining agents.

Magnesium orthosilicate can be doped with materials such as, chromium toform Cr:Forsterite. This crystal is a new tunable laser material thatfills the spectral void in the near-IR region. The tuning range for suchmaterial covers the important spectral range from 1130 to 1348 nm, whichprovides a minimal dispersion in optical fibers. The Cr:Forsterite lasereventually explores its niche applications for semiconductorcharacterization, eye-safe ranging, medical, industrial, and scientificresearch.

Magnesium orthosilicate, when doped with iron, forms magnesium ironsilicate. Its chemical name is Olivine and the chemical formula is(MgFe)₂SiO₄. The special class of Olivine is Peridot, which is green incolor and can be of a transparent gem variety.

In recent years, a considerable increase in the potential and actualuses of certain ceramics for structural, chemical, electrical andelectronic applications has occurred because of the strength, corrosionresistance, electrical conductivity and high temperature stabilitycharacteristics of these materials. Major applications for ceramicsinclude Si₃N₄/steel and Si₃N₄/Al joints in gas turbines and dieselengines, recuperators in heat exchangers, Si₃N₄/steel and Si₃N₄ /Tijoints in fuel cells, and ZrO₂/steel joints in friction materials forbearings, bushings, brakes, clutches and other energy absorbing devices.ZrO₂, Al₂O₃, and mullite/steel, Si₃N₄, and Al₂O₃ are also wear resistantmaterials. TiC/steel joints have been used as materials in cutting toolsand dies used in metal fabrication. SiC, Al₂O₃ and BN/Al steel jointshave also been used in space and military applications such as rocketnozzles, armor, missile bearings, gun barrel liners and as thermalprotection barriers in space vehicles. SiC/C, Al₂O₃/Si and Al₂O₃/Cu arebonded to Al joints are used in electronic devices and otherapplications.

To perform effectively and efficiently for many of these applications,ceramic components chosen often must coexist with or be bonded withmetallic components and form the system as a whole. Integration ofceramic-ceramic/metal hybrid parts into existing engineering designs cansignificantly enhance the performance of components.

The bonding or joining of ceramic parts or ceramic-metal components,however, presents a number of problems. For example, ceramic materialsmay differ, and ceramics and metals differ greatly in terms of modulusof elasticity, coefficient of thermal expansion, and thermaldiffusivity. Accordingly, large thermally induced mechanical stressesare set up in the joint regions during bonding. In the past, thisproblem has been overcome only with limited success in using commontechniques such as diffusion bonding, arc and oxyfuel fusion welding,brazing, soldering, and mechanical attachment techniques. Thus, whilediffusion bonding has proven useful for producing joints with goodelevated temperature properties, the practicality of this method islimited since it frequently requires vacuum and/or hot pressingequipment. Moreover, in the case of complicated shapes of the workpiecehaving non-planer mating surfaces, clamping of the workpiece to presspistons of a diffusion welding apparatus is needed to apply pressure tosurfaces to be joined, which is often expensive. The maximum size of thesurface area to be joined depends in such cases on the maximum forcethat can be brought to bear on the junction location. Alternatively,conventional fusion welding techniques create potentially criticalconditions, since the material at the interface is superheated for asubstantially long time, rises to accelerated reaction rates, and leadsto extensive interdiffusion of species. This situation results in theformation of an entirely different microstructure with degradedmechanical and chemical properties. In general, fusion welding may onlybe considered for materials with low stress applications.

While soldering and brazing techniques are relatively simple to carryout and may be conducted at lower temperatures, the procedure requireselaborate surface preparation, and most importantly, the joints producedare limited to applications, which do not involve high strength or hightemperature.

Other techniques, such as mechanical interlocking or electron-beamwelding, have their own peculiar drawbacks. For example, electron-beamwelding requires the use of a vacuum chamber. Additionally, it cannot beused with ease for dielectrics because of charge buildup on theinsulating ceramics.

For a more complete description of techniques for bonding ceramic partssee the following patents:

Schroeder et al. U.S. Pat. No. 4,420,352 discloses the joining ofceramic heat exchanger parts wherein the adjacent surfaces to be joinedare locally heated by RF heating.

Ebata et al. U.S. Pat. No. 4,724,020 discloses a similar process butwherein, the local heating is by high voltage torches. These disclosuresrequire specific heating and pre-heating apparatus adding to the cost ofthe joining process.

Ferguson et al. U.S. Pat. No. 4,767,479 discloses a method of bondinggreen (unfired) ceramic casting cores. The casting cores are made up ofceramic particles and a binder. The binder in the casting cores issoftened by applying a solvent and then ceramic filler particles areadded to at least one of the surfaces to be joined. Thereafter, theceramic casting cores are assembled until the solvent has evaporated.The bond strength of such joints are, however, dependent on the binder,and the use of solvents in the joining process may not beenvironmentally desirable.

Gat-Liquornik et al. U.S. Pat. No. 4,928,870 discloses the joining ofceramic parts by placing a metal foil or wire between the surfaces to bejoined under high pressure and then subjecting the metal foil or wire toa high current for a short period of time. However, the presence of ametal at the interface may adversely affect the physical properties,e.g., corrosion resistance, and thermal conductivity of the ceramicparts.

Iwamoto et al. U.S. Pat. No. 4,952,454 discloses the joining of ceramicparts wherein a paste including metals and metal oxides in an organicbinder is applied to the surfaces to be joined and then the assembly isheated to effect bonding. The organic binder upon pyrolysis can causepores at the interface, thereby, adversely affecting the bond strengthof the joined product.

Stover et al. U.S. Pat. No. 5,009,359 discloses diffusion welded jointsof ceramic parts that are produced by first plasma spraying a bondingmaterial at the seam of the mating pieces, followed by hot isostaticpressing. Such a two-step process adds to the complexity of the joiningoperation.

Dahotre et al. U.S. Pat. No. 5,503,703 discloses a ceramic joiningprocess that involves using an interlayer of a mixture of two materialsand reacting the materials with laser irradiation to form a thermallystable compound suitable for bonding the bodies together. The process,however, puts serious limitation on the choice of reacting materials,reaction products, reaction temperature and the ambient to avoiddeleterious effects on the physical properties of the bond. Moreover,the process requires the use of a high intensity laser adding to thecost of the joining process.

It is amply clear that although the prior art is replete with numerousceramic joining processes, there remains an important need for furtherinnovation for an improved method of ceramic-ceramic or ceramic-metalbonding. Most importantly, there is a need for such a method whichproduces a bond suitable for high temperature applications, usingmaterials of modest cost in a relatively simple and economic process.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method for bondinga metal to a ceramic or two ceramic materials.

It is a further object of the invention to eliminate many of the costlyprocesses used in bonding two similar or dissimilar materials.

These objects are achieved in a method of bonding a metal to a ceramicor two ceramic materials comprising the steps of:

(a) melting a polycrystalline magnesium orthosilicate compound; and

(b) applying the melted polycrystalline magnesium orthosilicate compoundbetween two materials which are metal with a ceramic, or two ceramicmaterials, or two metallic materials, bonding using the polycrystallinemagnesium orthosilicate to bond the two materials together.

It is an advantage of the present invention that melted polycrystallinemagnesium orthosilicate can be efficiently used to bond two similar ordissimilar materials.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a polycrystalline diffraction pattern collected using aRigaku RU-300 diffractometer, for Laponite clay deposited on MgO andsintered at 1500° C. for 2 hours (M—Mg₂SiO₄ diffraction peaks);

FIG. 2 shows a polycrystalline diffraction pattern collected using aRigaku RU-300 diffractometer, for Laponite clay mixed with ZrO₂ (70Laponite clay: 30 ZrO₂ wt:wt) deposited on MgO and sintered at 1500° C.for 2 hours (M—Mg₂SiO₄ peaks, Z(c)—Cubic ZrO₂ peaks);

FIG. 3 shows a polycrystalline diffraction pattern collected using aRigaku RU-300 diffractometer, for Laponite clay mixed with MgO (45Laponite clay:55 MgO wt:wt) deposited on α-Al₂O₃ and sintered at 1500°C. for 2 hours (M—Mg₂SiO₄, S—MgAl₂O₄, A—α-Al₂O₃); and

FIG. 4 shows a single crystalline diffraction pattern collected using aBruker GADDS microdiffractometer, for a single crystal removed from asample of Laponite clay mixed with ZrO₂ (70 Laponite clay:30 ZrO₂ wt:wt)deposited on MgO and sintered at 1500° C.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The silica based clay material used in this invention belongs to thegeneral class of phyllosilicates. A preferred choice includes smectiteclays, both natural and synthetic. One such material which can be usedin accordance with this invention is a commercially available syntheticsmectite clay which closely resembles the natural clay mineral hectoritein both structure and composition. Hectorite is a natural swelling claywhich is relatively rare and occurs contaminated with other mineralssuch as quartz which are difficult and expensive to remove. Syntheticsmectite is free from natural impurities, prepared under controlledconditions and commercially marketed under the tradename Laponite byLaporte Industries, Ltd of UK through its U.S. subsidiary, Southern ClayProducts, Inc. It is a 3-layered hydrous magnesium silicate, in whichmagnesium ions, partially replaced by suitable monovalent ions such aslithium, sodium, potassium and/or vacancies, are octahedrally bound tooxygen and/or hydroxyl ions, some of which may be replaced by fluorineions, forming the central octahedral sheet; such an octahedral sheet issandwiched between two tetrahedral sheets of silicon ions, tetrahedrallybound to oxygen.

There are many grades of Laponite such as RD, RDS, J, S, etc. each withunique characteristics and can be used for the present invention. Someof these products contain a polyphosphate peptizing agent such astetrasodium pyrophosphate for rapid dispersion capability;alternatively, a suitable peptizer can be incorporated into Laponitelater on for the same purpose. Typical chemical analyses of variousgrades of Laponite RDS and their physical properties, are disclosed inLaponite Product Bulletins.

TABLE 1 Typical Chemical Analysis Component Weight % SiO₂ 54.5 MgO 26.0Li₂O 0.8 Na₂O 5.6 P₂O₅ 4.1 Loss on ignition 8.0

TABLE 1 Typical Chemical Analysis Component Weight % SiO₂ 54.5 MgO 26.0Li₂O 0.8 Na₂O 5.6 P₂O₅ 4.1 Loss on ignition 8.0

Laponite separates into platelets of lateral dimension of 25-50 nm and athickness of 1-5 nm in deionized aqueous dispersions, commonly referredto as “sols.” Typical concentration of Laponite in a sol can be 0.1%through 10%. During dispersion in deionized water an electrical doublelayer forms around the clay platelets resulting in repulsion betweenthem and no structure build up. However, in sols containing electrolytesintroduced from tap water or other ingredients (e.g., some latexpolymers), the double layer can be reduced resulting in attractionbetween the platelets forming a “House of Cards” structure, and build-upof viscosity. Such sols can be easily coated on a substrate by suitablemeans.

In accordance with the present invention polycrystalline magnesiumorthosilicate is made by the following steps:

(a) contacting a silica based clay material with magnesium oxide,preferably the weight ratio of silica based clay:MgO is in the range of10:90 to 90:10; and

(b) sintering the magnesium oxide contacting the silica based claymaterial to cause a solid state diffusion of the magnesium oxide intothe silica based clay material to thereby produce polycrystallinemagnesium orthosilicate. In the examples which follow the silica basedclay material is Laponite.

The diffraction pattern shown in FIG. 1 indicates that the Laponiteclay—MgO mixture has converted to Mg₂SiO₄, an olivine-type mineral knownas forsterite, upon sintering the mixture at 1500° C. for 2 hours. Thepresence of multiple diffraction peaks is consistent with this samplebeing polycrystalline.

The diffraction pattern shown in FIG. 2 indicates that when a Laponiteclay—ZrO₂ mixture deposited on MgO is sintered at 1500° C. for 2 hours,MgO reacts with the clay to form Mg₂SiO₄ (Forsterite) and the MgO reactswith the ZrO₂ generating cubic zirconia. The presence of multiplediffraction peaks is consistent with this sample being polycrystalline.

The diffraction pattern shown in FIG. 3 indicates that when a Laponiteclay—MgO mixture is deposited on Al₂O₃ and sintered to 1500° C. for 2hours, the MgO reacts with the clay to form Mg₂SiO₄ (Forsterite) and theMgO reacts with the Al₂O₃ generating MgAl₂O₄ (spinel). Unreacted Al₂O₃from the substrate was also detected in the sample. The presence ofmultiple diffraction peaks is consistent with this sample beingpolycrystalline.

In accordance with the present invention a method of making singlecrystalline magnesium orthosilicate includes the steps of:

(a) forming a mixture of silica based clay and an oxide ceramic;

(b) contacting the silica based clay and oxide ceramic mixture withmagnesium oxide; and

(c) sintering the magnesium oxide contacting the silica based claymaterial to cause a solid state diffusion of the magnesium oxide intothe silica based clay and the oxide ceramic mixture to form a compoundand melting the compound to thereby produce single crystalline magnesiumorthosilicate. In the example which follows, the silica based claymaterial is Laponite.

The diffraction pattern shown in FIG. 4 is from a single crystalextracted from a sample of Laponite clay—ZrO₂ deposited on MgO andsintered at 1500° C. for 2 hours. The spot pattern is a Laue-typepattern indicating that the specimen analyzed was in fact a singlecrystal (A polycrystalline sample would result in a series ofdiffraction rings). Integration of the spot positions and intensitiesindicate that the single crystals generated in this manner areMg₂SiO₄(Forsterite).

In accordance with the invention it has been found that magnesiumothosilicate phase formation at a relatively lower temperature in therange of 1300° C. to 1600° C., preferably at 1500° C. is a significantphenomenon compared to the reported higher temperature of its meltformation at 1900° C.

Instead of using silicates, synthetic clay material, such as Laponiteclay was mixed with magnesium oxide, either in the powder or bulk formand the mixture of these two ingredients were heated in the range of1300-1600° C., preferably at 1500° C. for 2 hours. Depending on thesubstrates used for the chemical reactions to occur at high temperature,various phases such as cubic zirconia and spinel along withpolycrystalline magnesia orthosilicate are formed as illustrated inFIGS. 1 to 3.

In certain instances, single crystal magnesia orthosilicate phase hasformed as shown in FIG. 4. This is presumably due to use of higherproportions of clay which results in the formation of a flux whichallowed the growth of single crystals from the melt. It has beendetermined that a clay content more than 65 weight percent is necessaryfor single crystal formation. These single crystals can be found to bepresent with polycrystalline phases.

It has been found that molten polycrystalline magnesium orthosilicatecan be effectively used to bond two materials which include a metal witha ceramic or two ceramic oxide materials. Steps involved in making thepolycrystalline magnesium orthosilicate are as follows:

(a) contacting a silica based clay material with magnesium oxide; and

(b) sintering the magnesium oxide contacting the silica based claymaterial to cause a solid state diffusion of the magnesium oxide intothe silica based clay material to thereby produce polycrystallinemagnesium orthosilicate compound. In order to bond two materialstogether which can be either a metal and a ceramic or two similar ordissimilar ceramics the following steps are used:

(a) melting the polycrystalline magnesium orthosilicate compound; and

(b) applying the melted polycrystalline magnesium orthosilicate compoundbetween two materials which are metal with a ceramic or two ceramicmaterials bonding using the polycrystalline magnesium orthosilicate tobond the two materials together.

Molten magnesium orthosilicate was found to be an effectivejoining/bonding compound for a metal and a ceramic, or for two ceramicmaterials, or two metallic materials, which are either similar ordissimilar. Noble metals and their alloys, Pt, Au, Pd, Ir, Pt—Rh, Pt—Ir,Au—Pd for example, and transition metals and their alloys, Fe, Ni, Ti,Fe—Ni, Ni—Co for example, are particularly suitable for metal to metal(or alloy to alloy) bonding. For specific examples, the followingmaterials were bonded together ZrO₂/ZrO₂, ZrO₂/MgO, MgO/MgO, Al₂O₃/ZrO₂,Al₂O₃/Al₂O₃, Al₂O₃/MgO, Al₂O₃/Platinum ZrO₂/Platinum. The integrity ofthe joints was evaluated by standard drop test methods and themicrostructures of the bonded regions were examined. From these tests,it was determined that the joints were of acceptable quality.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention.

What is claimed is:
 1. A method of bonding a metal to a ceramic, or twoceramic material, or two metallic material, comprising the steps of: (a)melting a polycrystalline magnesium orthosilicate compound; and (b)applying the melted polycrystalline magnesium orthosilicate compoundbetween two materials which are metal with a ceramic, or two ceramicmaterials, or two metallic materials, using the polycrystallinemagnesium orthosilicate to bond the two materials together.
 2. A methodof making polycrystalline magnesium orthosilicate and using it to bondtwo similar or disimilar materials which include a metal with a ceramic,or two ceramic oxide materials, or two metallic materials comprising thesteps of: (a) contacting a silica based clay material with magnesiumoxide; (b) sintering the magnesium oxide contacting the silica basedclay material to cause a solid state diffusion of the magnesium oxideinto the silica based clay material to thereby produce polycrystallinemagnesium orthosilicate compound; (c) melting the polycrystallinemagnesium orthosilicate compound; and (d) applying the meltedpolycrystalline magnesium orthosilicate compound between two materialswhich are metal with a ceramic, or two ceramic materials, or twometallic materials using the polycrystalline magnesium orthosilicate tobond the two materials together.
 3. The method of claim 2 wherein thetwo ceramic materials are either the same or different materials.
 4. Themethod of claim 3 wherein the ceramic material is selected from thegroup consisting of zirconia, magnesia, alumina and composites thereof.5. The method of claim 2 wherein the metal is selected from the groupconsisting of noble metals and their alloys, and transition metals andtheir alloys.
 6. The method of claim 2 wherein the sintering temperatureis a range of 1300 to 1600° C.